Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as an engine speed/load window, as well as various other operating conditions including operator torque demand. Conventional VDE control systems may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. Newer skip-fire or rolling VDE systems may be configured to activate/deactivate individual cylinders on an ongoing basis to provide a specific firing pattern based on a designated control algorithm.
During partial cylinder operation, the active cylinders operate at a higher load than they otherwise would for the overall torque level generated by the engine. During the higher load conditions, engine knock may be more prevalent. Engine knock, if left unabated, may lead to engine degradation. As one example, ignition timing may be retarded in the engine cylinders to abate knock. However, ignition timing retard may negatively impact fuel economy. Thus, methods to detect and mitigate knock in VDE systems are desirable, and in particular methods to accurately detect which cylinder is knocking.
One example approach for detecting and abating knock in a VDE system is described by Glugla in U.S. Patent Application Publication No. 2014/0350823. Therein, when knock is detected during partial cylinder operation, only the cylinder determined to be undergoing knock is subjected to ignition timing adjustment, as the crank angle between combustion events may be large enough to accurately identify the knocking cylinder. During full cylinder operation, when knock is detected, multiple cylinders may undergo ignition timing adjustment, thereby increasing the odds that knock occurring in the knocking cylinder will be abated during conditions where it may be difficult to detect which cylinder is knocking.
However, the inventors herein have recognized potential issues with such systems. As one example, identification of the knocking cylinder usually relies on engine position data output by a crankshaft position sensor. These crankshaft position sensors typically utilize a toothed wheel coupled to the crankshaft to measure engine position. However, over time the size and spacing of the teeth on the wheel may change, as well as the shape of the wheel itself. These changes may cause the crankshaft position sensor to output inaccurate engine position data, leading to inaccurate determinations of which cylinder is knocking. Thus, spark timing retard may be applied to the wrong cylinder, negatively impacting fuel economy without abating knock.
Accordingly, an approach is provided herein to at least partly address these issues. In one example, a method includes during fuel-cut operation, heating an engine cylinder and upon fuel reactivation, re-learning a crankshaft sensor profile responsive to lack of knock in the heated cylinder. In this way, a cylinder that is determined to be persistently knocking (e.g., knocking despite application of spark timing retard) may be induced to knock by heating the cylinder during a fuel-cut engine operating condition, such as deceleration fuel shut off. If the cylinder is induced to knock, the cylinder may be confirmed as persistently knocking due to soot build-up in the cylinder, for example. However, if the cylinder is not induced to knock upon being heated and reactivated, but a different cylinder of the engine is induced to knock upon being heated and reactivated (e.g., a cylinder adjacent in the engine firing order), the crankshaft position sensor may be implicated as having undergone a change in performance (e.g., a change in tooth spacing and/or size).
In one example, the cylinders may be induced to knock during the fuel-cut engine operation one by one by utilizing the selective cylinder deactivation mechanisms to hold the intake and exhaust valves of the cylinders closed during the fuel-cut operation. Continued rotation of the crankshaft causes the pistons to move up and down in their respective cylinders, leading to heating of the gasses trapped in the cylinders. The cylinders are then reactivated one at a time and output from the knock sensor is monitored to determine if any knocking occurs. In this way, knocking cylinders may be identified without relying on the crankshaft position sensor data. If the crankshaft position sensor is determined to be outputting inaccurate engine position data, the tooth profile of the crankshaft position sensor wheel may be relearned.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.